1. Field of the Invention
The present invention generally relates to a field of image processing, and more particularly, to a field of an image forming or displaying device applicable to a laser printer, a digital copying machine, a color laser printer and a facsimile device.
2. Description of the Related Art
Conventionally, the dither method, the density pattern method, and the random dither method are generally used as a middle-tone reproducing method in an image-forming device, such as a laser printer, a digital copying machine, a color laser printer and a facsimile device.
The dither method expresses the gradation with a plurality of pixels, and expresses colors with the combination thereof in a color image. The dither method used in a general printing is excellent in granularity, and expresses a smooth middle-tone image. In the so-called area gradation methods represented by the dither method, a resolution is deteriorated in exchange for obtaining a gradation. Additionally, with respect to a print image like dots, the dither method producing periodic images tends to generate moirés.
The random dither method is a method for expressing gradation while maintaining resolution. The random dither method is suitable to a reproduction of a text image because the random dither method can obtain a resolution faithful to the original image. However, the random dither method may possibly produce a peculiar texture with respect to a halftone image such as a photographic part because isolate dots are scattered or arranged in irregular connections to deteriorate granularity. Especially in an electrophotographic printer, since an image is formed by isolate dots, a deterioration of granularity or a banding tends to occur in the unstable image due to uneven density.
By the way, regarding an image-forming device and the like, it may sometimes be desired to convert multivalued image data of low resolution into binary image data of high resolution. Applying the random dither method to this case conceivably involves a method of providing the multivalued image data with high resolution prior to a random dither process. However, the random dither process is a complicated and time-consuming process because the random dither process performs a product-sum calculation to diffuse quantization errors of peripheral pixels. Specifically, the number of pixels in a unit area increases as the resolution of the binary image data becomes higher, and the calculation amount and the processing time of the random dither process increase in approximate proportion to the number of pixels in a unit area. Thus, for example, when the resolution varies from 600 dpi to 1200 dpi and 2400 dpi, the number of pixels in a unit area increases 4 times and 16 times, respectively, in proportion to the square of the resolution; therefore, the calculation amount and the processing time have to increase largely. Hence, in order to maintain productivity, the process needs to be performed at a higher speed to compensate for the increased calculation amount and restrain the increase in the processing time.
An effective method to restrain the increase in the processing time is quantizing the multivalued image data of low resolution by the random dither process, and thereafter converting the multivalued image data of low resolution into the binary image data of high resolution, because this method reduces a calculation amount and a circuit scale required for the random dither process, compared to quantizing image data of high resolution.
An example of a device realizing such a conventional technology can be found in Japanese Laid-Open Patent Application No. 07-295527. In this technology, after multivalued image data of low resolution (600 dpi) is quantized in multivalues by the random dither process, the multivalued image data is converted into binary image data of high resolution (1200 dpi) by the density pattern method or the dither method.
Another example of a method realizing such a conventional technology can be found in Japanese Laid-Open Patent Application No. 11-155064. In this technology, after data of low resolution (600 dpi) is binarized by the random dither process, the data is converted into binary image data of high resolution (1200 dpi) by pattern matching.
Objects of the above-mentioned device (Japanese Laid-Open Patent Application No. 07-295527) are to increase the processing speed, reduce the calculation time, and prevent the circuit scale from increasing due to a buffer memory, while achieving binary image signal having sufficient gradations at a high speed. Another object of this device is to inhibit moirés and Rosetta patterns. In order to achieve these objects, this conventional device performs a gradation process reducing the number of gradations per pixel by the multivalued random dither process, and based on the result thereof, performs another gradation process, i.e., the density pattern method. However, since dots are arranged by a simple method, such as the density pattern method or the dither method, it is difficult to increase the quality of an image formed by a printer performing a high-density writing of 1200 dpi or more, especially an electrophotographic printer in which dots are reproduced less clearly as the dots become denser. Additionally, arranging dots by the density pattern method or the dither method inflicts periodicity on an image, and may possibly generate moirés.
The above-mentioned method (Japanese Laid-Open Patent Application No. 11-155064) attempts to make a large improvement in granularity at a highlight part by using a little buffer memory and performing a few processes. However, no particular change or remarkably improved quality can be observed in an output image, compared to the random dither process of 600 dpi.